ECR CVD method for forming BN films

ABSTRACT

A method of forming a boron nitride containing film on a substrate is disclosed. The method includes disposing a substrate in a reaction chamber, inputting a reactive gas comprising boron and nitrogen into the reaction chamber, exciting the reactive gas in the reaction chamber by applying a DC biased, RF electric field thereto in the presence of a magnetic field, and depositing the boron nitride containing film on the substrate.

This application is a continuation of Ser. No. 07/431,870, filed Nov. 6,1989, now abandoned, which was a divisional of application Ser. No.07/414,593, filed Sept. 26, 1989, now U.S. Pat. No. 5,013,579, which wasa continuation of application Ser. No. 07/154,286, filed Feb. 10, 1988,now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a microwave enhanced CVD method for forming acarbon film and its products.

Recently, ECR CVD has attracted the interests of researchers as a newmethod of manufacturing thin films, particularly amorphous thin films.For example, Matsuo et al discloses one type of such an ECR CVDapparatus in U.S. Pat. No. 4,401,054. This recent technique utilizesmicrowaves to energize a reactive gas into a plasma state by virtue of amagnetic field which functions to pinch the plasma gas within theexcitation space. With this configuration, the reactive gas can absorbthe energy of the microwaves. A substrate to be coated is locateddistant from the excitation space (resonating space) for preventing thesame from being spattered. The energized gas is showered on thesubstrate from the resonating space. In order to establish an electroncyclotron resonance, the pressure in a resonating space is kept at1×10⁻³ to 1×10⁻⁵ Torr at which electrons can be considered asindependent particles and resonate with a microwave in an electroncyclotron resonance on a certain surface on which the magnetic fieldtakes a particular strength required for ECR. The excited plasma isextrated from the resonating space, by means of a divergent magneticfield, to a deposition space which is located distant from theresonating space and in which is disposed a substrate to be coated.

In such a prior art method, it is very difficult to form a thin film ofa polycrystalline or single-crystalline structure, so that currentlyavailable methods are almost limited to processes for manufacturingamourphous films which have lower hardness. Also, high energy chemicalvapor reaction is difficult to take place in accordance with such aprior art and therefore a diamond film or other carbon films having highmelting points, or uniform films on uneven surface, such as exteriors ofthe parts of watchs, which have depressions and caves can not be formed.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to realize a light, beautifuland weariless watch.

The carbon coating is implemented for several purposes. When the partsof gears or screws as mechanical parts for watches are coated, thecarbon coating function to reinforce the strength of the parts and endowthe parts with the resistance to abrasion. On the other hand, when theexperior of watches are coated, the carbon coatings become decorations.

According to one aspect of the invention, in addition to a carboncompound, nitrogen and/or a nitrogen compound gas is inputted to thereaction chamber. The inputted nitrogen functions to prevent lattecedefects from growing by virtue of external or internal stress. When aboron compound is also inputted together with the nitrogen compound, theadhesivity of carbon deposited is improved. Boron nitride appears to bethe binder between the carbon and the underlying substrate to be coatedsuch as parts of watch. Preferably, carbon and boron nitride aredeposited on the substrate in the form of crystalline grain particles ora layer containing nitrogen and boron at less than 10%.

According to another aspect of the invention, a new CVD process has beenculminated. The new process utilizes a mixed cyclotron resonance whichwas introduced firstly by the inventors. In the new type of excitingprocess, a sonic action of reactive gas itself must be taken intoconsideration as a non-negligible perturbation besides the interactionbetween respective particles of the reactive gas and magnetic field andmicrowave, and therefore charged particles of a reactive gas can beabsorbed in a relatively wide resonating space. Preferably, the pressureis maintained higher than 3 Torr. For the mixed resonance, the pressurein a reaction chamber is elevated 10² -10⁵ times as high as that ofprior art. For example, the mixed renonance can be established byincreasing the pressure after ECR takes place at a low pressure. Namely,first a plasma gas is placed in ECR condition at 1×10⁻³ to 1×10⁻⁵ Torrby inputting microwave under the existence of magnetic field. Then areactive gas is inputted into the plasma gas so that the pressure iselevated to 0.1 to 300 Torr and the resonance is changed from ECR to MCR(Mixed Cyclotron Resonance). The MCR is a new type of resonanceutilizing the whistler mode. Carbon can be decomposed and undergo anecessary reaction at only such a comparatively high pressure. Inprocess, diamond is likely to grow selectively on convexies.

Although carbon is deposited also in an amorphous phase when diamond isprefered, hydrogen in a plasma state eliminates preferentially amorphouscarbon by etching, remaining crystalline carbon.

It has been found that the hardness of the diamond formed by the presentinvention is 1.3 to 3.0 times as high as that of diamond which has beenmade by prior art vapor phase method. In what follows, the term, "film,"is used in a broad meaning. If a number of diamond particles are finelydistributed on a furface, we call such a diamond coating as a "film." Ofcourse, a uniform and continuous amorphous layer is called a "film."

When a number of plastic gear-wheels are coated with diamond film, e.g.,1-10 micron thick in accordance with the present invention, a new kindof gear-wheel is obtained which is characterized by a light weight, ahigh wear resistance, a smooth surface and a low price.

When the parts of a wrist watch are coated with carbon film, such partscan be made of plastics and alminium or its alloy having attractiveprocessability so that the wrist watch is strongly formed and lightweighted. Especially, electric field tends to be concentrated at thecorners of the parts, and therefore such corners particularly exposed toexternal impacts can be coated with a thicker carbon film by a factor oftwo in comparison with flat surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view showing a CVD apparatus in accordancewith the present invention.

FIG. 2(A) is a graphical diagram showing the plofile of theequipotential surfaces of magnetic field in cross section in accordancewith a computor simulation.

FIG. 2(B) is a graphical diagram showing the strength of electric fieldin accordance with a computor simulation.

FIGS. 3(A) and 3(B) are graphical diagrams showing equipotentialsurfaces in terms of magnetic field and electric field of microwavepropagating in a resonating space respectively.

FIG. 4 is a cross sectional view showing another CVD apparatus fordepositing a carbon layer in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an embodiment of the invention is illustrated. Inthe figure, the apparatus comprises a reaction chamber in which a plasmagenerating space 1 and an auxiliary space 2 are defined and which can beheld at an appropriate pressure, a microwave generator 4,electro-magnets 5 and 5' which are supplied with an electric power froma power supply 25, and a water cooling system 18. The plasma generatingspace 1 has a circular cross section. In the plasma generating space 1,a hollow cylinder 10' having inward-turned brims is rotatably supportedin the space so that a microwave emitted from the microwave generator 4passes through the cylinder along its axis. The cylinder 10' is made ofa stainless steel or a quartz and turned by means of a motor 16 througha gear. Provided for evacuating the reaction chamber is a evacuatingsystem comprising a turbo molecular pump 8 and a rotary pump 14 whichare connected with the reaction chamber through pressure controllingvalves 11, 12 and 13. The process with this appratus is carried out asfollow.

Objects 10 to be coated with carbon film are parts of watches such asframes made of aluminium alloys or plastics, windows made of glasses orplastics, or timing mechanism parts such as gear-wheels with 3-10 mm indiameter and 0.2-2.0 mm in thickness. The objects 10 are put in thecylinder 10' and turned at 0.1-10 rpm during process. At the same time,the objects 10 are heated to a certain elevated temperature at which theobjects are not damaged. For plastic objects, the elevated temperatureis, e.g., 150°-300° C.; for aluminium or its alloy objects, e.g.,400°-700° C.; for iron, stainless steel or other metallic objects, e.g.,700°-1000° C. As the temperature elevates, the hardness of the coatingbecomes high and the proportion of diamond increases. The cylinder 10',although the means is not illustrated in the figure, is shaken bymicro-vibration of 100 Hz-10 KHz. By the turning and the vibration, thesurfaces of the objects exposed to the reactive gas are always switchingduring process. The reaction chamber is evacuated by the turbo molecularpump 8 and the rotary pump to 1×10⁻⁶ Torr or lower. Then, argon, heliumor hydrogen as a non-productive gas is introduced to the reactionchamber from a gas introducing system 6 at 30 SCCM, and a microwave of2.45 GHz is emitted from the microwave generator at 500 W through amicrowave introduction window 15 to the plasma generating space 1 whichis subjected to an magnetic field of about 2K Gauss induced by themagnets 5 and 5'. The pressure of the non-productive gas is 1×10⁻⁴ Torr.A plasma is generated in the space 1 at a high density by the energy ofthe microwave. The surfaces of the objects 10 is cleaned by high energyelecrons and non-productive atoms. In addition to the introduction ofnon-productive gas, C₂ H₂, C₂ H₄ and/or CH₄ are introduced at 200 SCCMthrough a introduction system 7. At the same time, a large amount ofhydrogen is introduced into the reaction chamber so that the carboncompound gas is diluted with the hydrogen to 0.1-2.0%. The pressure ofthe reaction chamber is maintanined at 0.1-300 Torr, preferably 3-30Torr, e.g., 10 Torr. By virtue of the comparatively high pressure, theproduct can be deposited at a high speed and spread over widely in thechamber. The reactive gas is excited by the energy of microwave in thesame manner as carried out with the non-productive gas explained in theforegoing description. As a result of a mixed resonance, carbon isdeposited in the form of a diamond film or an i-carbon (insulated carbonconsisting of crystalline particles) film on the objects 10.

FIG. 2(A) is a graphical diagram showing the distribution of magneticfield on the region 30 in FIG. 1. Curves on the diagram are plottedalong equipotential surfaces and given numerals indicating the strengthson the respective curves of the magnetic field induced by the magnets 5and 5' having a power of 2000 Gauss. By adjusting the power of themagnets 5 and 5', the strength of the magnetic filed can be controlledso that the magnetic field becomes largely uniform over the surface tobe coated which is located in the region 100 where the magnetic field(875±185 Gauss) and the electric field interact. In the diagram, areference 26 designates the equipotential surface of 875 Gauss at whichECR (electron cyclotron resonance) condition between the magnetic fieldand the frequency of the microwave is sutisfied. Of course, inaccordance with the present invention, ECR can not be established due tothe high pressure in the reaction chamber, but instead a mixed cyclotronresonance (MCR) takes place in a broad region including theequipotential surface of the ECR condition. FIG. 2(B) is a graphicaldiagram of which the X-axis corresponds to that of FIG. 2(A) and showsthe strength of electric field of the microwave in the plasma generatingspace 1. The strength of the electric field takes its maximum value inthe regions 100 and 100'. However, in the region 100', it is difficultto heat the substrate 10' without disturbing the propagation of themicrowave. In other region, a film is not uniformly deposited, butdeposited the product in the form of a doughnut. It is for this reasonthat the substrate 10 is disposed in the region 100. The plasma flows inthe lateral direction. According to the experimental, a uniform film canbe formed on a circular substrate having a diameter of up to 100 mm.Preferably, a film is formed in the chamber on a circular substratehaving a diameter of up to 50 mm with a uniform thickness and a uniformquality. When a larger substrate is desired to be coated, the diameterof the space 1 can be sized double with respect to the verticaldirection of FIG. 2(A) by making use of 1.225 GHz as the frequency ofthe microwave. FIGS. 3(A) and 3(B) are graphical diagrams showing thedistributions of the magnetic field and the electric field due tomicrowave emitted from the microwave generator 4 on a cross section ofthe plasma generating space 1. The curves in the circles of the figuresare plotted along equipotential surfaces and given numerals showing thestrength. As shown in FIG. 3(B), the electric field reaches its maxmumvalue at 25 KV/m.

The diffraction images of films formed in accordance with the presentinvention were obtained. As results, halo patterns were obserbedtogether with spots indicating the existence of diamond. When the filmwas deposited at a substrate temperature as low as 350° C., a halopattern which is peculier to amorphous structure was observed. On theother hand, clear spots indicating the existence of diamond appeared onthe deffraction pattern of the film deposited at a substrate temperature800° C. or higher. When the film was deposited at an intermediatetemperature, the carbon film became i-carbon film which is the mixtureof amorphous carbon and micro-crystalline carbon. Further, the filmswere doposited at 150°-350° C. by virtue of different input powers. Whenthe power of microwave inputted was 1.0 KW, a halo pattern and spots dueto the existence of diamond were simultaneously observed indicating ani-carbon structure. The halo patterns gradually disappeared as themicrowave power elevates, and when the power reaches a high level notlower than 1.5 KW the film became rich in diamond structure. On thisexperimental, the carbon films contain hydrogen at 1-30 at %. Stillfurther, the films were deposited at 700° C. by virtue of differentinput powers. As the microwave power elevated from 500 W, the halopattern gradually disappeared, and when the power reached 700 W orhigher, diamond structure prevailed in the film.

The pressure in the reaction chamber is chosen at that required for ECRcondition, so that a preliminary plasma discharge takes place. While thedischarge continues, the pressure is changed to 1 Torr to 3×10³ Torrwhere a mixed resonance takes place with a plasma of which particleshave a mean free path of 0.05 mm to several milimeters, normally notmore than 1 mm.

Next, another deposition method in accordance with the present inventionwill be described. The deposition apparatus used for the precedingembodiment can be used also for this embodiment.

A number of objects 10 such as plastic gear wheels are placed in thecylinder 10', and the reaction chamber is evacuated to 1×10⁻⁶ Torr or ahigher vacuum condition. Then, hydrogen gas in introduced from a gasintroducing system 6 at 30 SCCM, and a microwave of 500 Watt at 2.45 GHzis emitted from the microwave generator 4 thorugh a microwaveintroduction window 15 to the plasma generating space 1 which issubjected to an magnetic field of about 2K Gauss induced by the magnets5 and 5'. The hydrogen is excited into a high density plasma state inthe space 1 at 1×10⁻⁴ Torr by the energy of the microwave. The surfacesof the objects 10 are cleaned by high energy elecrons and hydrogenatoms. In addition to the introduction of the hydrogen gas, a carboncompound gas as the productive gas such as C₂ H₂, C₂ H₄, CH₃ OH, C₂ H₅OH or CH₄ are inputted at 30 SCCM through an introduction system 7. Inthis process, the productive gas is diluted with hydrogen at asufficiently thin density, e.g., 0.1 to 5%. Further in addition to this,a nitrogen or its compound gas, such as ammonia or nitrogen gas, isinputted to the reation chamber from the introduction system. Theproportion of the nitrogen compound gas to the carbon compuond gas is0.1%-5%. Then, the pressure in the reaction chamber is maintained at 0.1Torr-300 Torr, preferably 3-30 Torr, e.g., 1 Torr. By increasing thispressure in the reaction chamber, it is possible to make high thedensity of the productive gas and, therefore, faster the growth rate ofthe product. Namely, carbon atoms are excited in a high energy conditionso that the objects 10 disposed in the cylinder 10' is coated withcarbon in the form of a film made of i-carbon or diamond having 0.1 to100 microns in grain diameter. The deposited carbon contains nitrogen at0.01-1 weight %.

Next, a further embodiment will be described. A objects 10 are disposedin the cylinder 10', and the reaction chamber is evacuated to 1×10⁻⁶Torr or a higher vacuum condition. Then, hydrogen gas is introduced froma gas introducing system 6 at 300 SCCM, and a microwave of 1 Kilo Wattat 2.45 GHz is emitted from the microwave generator 4 thorugh amicrowave introduction window 15 to the plasma generating space 1 whichis subjected to an magnetic field of about 2K Gauss induced by themagnets 5 and 5'. The hydrogen is excited into a high density plasmastate in the space 1 by the energy of the microwave. The surfaces of theobjects 10 are cleaned by high energy elecrons and hydrogen atoms. Inaddition to the introduction of the hydrogen gas, a cabon compound gasas the productive gas such as C₂ H₂, C₂ H₄, CH₃ OH, C₂ H₅ OH or CH₄ areinputted at 3 SCCM through an introduction system 7. In this process,the productive gas is diluted with hydrogen at a sufficiently thindensity, e.g., 0.1 to 15%. Further in addition to this, a nitrogencompound gas such as ammonia, NO₂, NO, N₂ or nitrogen gas, and B₂ H₆ orBF₃ are inputted to the reation chamber from the introduction systems 7and 8 respectively at B/N=1. The proportion of B₂ H₆ (BF₃)+NH₃ to thecarbon compound gas is 1%-50%. Then, the pressure in the reactionchamber is maintained at 1 Torr-760 Torr, preferably higher than 10 Torror 10-100 Torr, e.g., 30 Torr. By increasing this pressure in thereaction chamber, it is possible to make high the density of theproductive gas and, therefore, faster the growth rate of the product.Namely, the objects 10 disposed in the cylinder 10' are coated withcarbon containing nitrogen and boron (or in the form of boron nitride).The product includes carbon and boron nitride as the main components,the sum of whose proportions is at least 90%.

On the electron beam diffraction image of the thin film produced inaccordance with the above procedure, observed are spots indicating thepresence of polycrystalline boron nitride and crystal carbon, i.e.,diamond (single-crystalline particles). Namely, the film is made of themixture of boron nitride and diamond. As the microwave power isincreased from 1 KW to 5 KW, the proportion of diamond in the filminceases.

When BF₃ and/or NF₃ is used as the boron and/or nitrogen source, theplasma gas becomes containing fluorine and which fluorine functions toeliminate impurity residing on the surface to be coated by etching.

For reference, a film formation process was performed in the same manneras in the above but without using a magnetic field. As a result, agraphite film was deposited.

By a similar process, amorphous or microcrystalline film can also bedeposited by appropriately selecting the deposition condition. Anamorphous film is deposited when carbon compound gas is diluted with thelarger amount of hydrogen gas, when the input power is comparativelysmall and when the process temperature is comparatuvely low.

It is a significant feature of the invention that the carbon formed inaccordance with the invention has a very high hardness irrespective ofwhether the carbon is amorphous or crystalline. The Vickers hardness is4500-6400 Kg/mm², e.g., 2000 Kg/mm². The thermal conductivity is notlower than 2.5 W/cm deg, e.g., 5.0-6.6 W/cm deg.

The present invention can be applied for the formation of carbon bymeans of glow or arc discharge enhanced CVD caused by an r.f. power.FIG. 4 is a cross sectional view showing a CVD apparatus for depositionby virtue of an r.f. power. In the figure, the apparatus comprises areaction chamber 101, a loading chamber 103, a rotary pump 105 forevacuating the loading chamber 103, a turbo molecular pump 107associated with a rotary pump 109 for evacuating both the reactionchamber 101 and the loading chamber 103, a gas feeding system 127 forinputting process gas such as reactive gas or dopant gas through anozzle 129, a substrate holder 111 for supporting objects 113,electrodes 115 disposed opposite to the holder 111, an RF power supply117 consisting of a radiofrequency power source 119 associated with amatching circuit 121 and a DC bias circuit 123 for supply an r.f. powerbetween the electrodes 115 and the substrate holder 111, and a halogenlamp heater 125 with a quartz window 129 for heating the objects 113.The deposition process for coating the objects 113 with a carbon film isas follow.

After disposing the objects 113 in the reaction chamber 101 through agate 129, a reactive gas composed of a gaseous carbon compound such asCH₄, C₂ H₄ and C₂ H₂, and a dopant gas such as nitrogen, a nitrogencompound gas and a boron compound gas if necessary were inputted to thereaction chamber at 1×10⁻³ to 5×10⁻¹ Torr. The carbon compound gas wasdiluted with hydrogen at 50 mol %. At the same time, the objects 113were heated to not higher than 450° C. by means of the heater 125. Inthis condition, a vapor reaction was initiated by means of r.f. powerinputted from the power supply 117. The r.f. power was 50 w to 1 KW(0.03 to 3.00 W/cm²) at 13.56 MHz superimposed on an DC bias voltage of-200 V to +400 V. Then, carbon films were deposited on the objects 113at a growth rate of 150 Å/min. The carbon film looked like an amorphousstructure rather than a cystalline structure. Despite the amorphousstructure, the hardness was measured as high as that of a diamond film.The Vickers hardness thereof was 4500-6400 Kg/mm², e.g., 2000 Kg/mm². Sowe call it "diamond-like carbon" or DLC for short.

In accordance with the present invention, a super lattice structure canbe also formed. A boron nitride (BN) thin film is deposited in the sameway as illustrated in the above but without using carbon compound gas. Acarbon thin film and a BN thin film are deposited in turn may times sothat a super lattice structure is sttached on a substrate.

The invention should not limited to the above particular embodiments andmany modifications and variations may cause to those skilled in the art.For example, it has been proved effective to add aluminum or phosphorousinto carbonat 0.001 to 1 weight %.

I claim:
 1. A method of forming a boron nitride containing filmcomprising the steps of:disposing a substrate in a reaction chamber;inputting into said reaction chamber a reactive gas comprising boron,fluorine and nitrogen; inputting a microwave into said reaction chamber;generating a magnetic field in said reaction chamber, said magneticfield having a configuration such that a condition for causing acyclotron resonance can be satisfied at a position in said reactionchamber; exciting said reactive gas in said reaction chamber by saidmagnetic field; and depositing said boron nitride containing film onsaid substrate, wherein said substrate is located at approximately saidposition in said reaction chamber.
 2. A method as in claim 1 where thefrequency of said microwave is 2.45 GHz and said substrate is locatedwhere the strength of said magnetic field is 875±175 Gauss.
 3. A methodas in claim 1 where said substrate is made of an aluminum alloy.
 4. Amethod as in claim 1 wherein said substrate has at least one flatsurface and at least one corner.
 5. A method as in claim 4 where athickness of said boron nitride containing film at said corner is abouttwice that of the film on the flat surface.
 6. A method of forming aboron nitride containing film on a substrate, said method comprising thesteps of:disposing said substrate in a reaction chamber; inputting intosaid reaction chamber a reactive gas comprising boron, and nitrogen;exciting said reactive gas in said reaction chamber by applyingmicrowave electromagnetic energy thereto in the presence of a magneticfield in the pressure range of 0.01-300 Torr to cause cyclotronresonance in the reaction chamber; and depositing said boron nitridecontaining film on said substrate.
 7. A method as in claim 6, where saidsubstrate is made of a plastic.
 8. A method as in claim 6, where saidsubstrate is made of an aluminum alloy.
 9. A method as in claim 6, wheresaid substrate has at least one flat surface and at least one corner.10. A method as in claim 9, where a thickness of said boron nitridecontaining film at said corner is about twice that of the film on theflat surface.
 11. A method as in claim 6, where said reactive gasfurther includes fluorine.
 12. A method of forming a stacked structurefor a substrate, said method comprising the steps of:a) disposing asubstrate in a reaction chamber; b) inputting into said reaction chambera first reactive gas comprising one of (a) carbon or (b) boron andnitrogen; c) exciting said first reactive gas in said reaction chamberby applying thereto a DC biased, RF electric field; d) depositing a filmformed from said first reactive gas; e) inputting into said reactionchamber a second reactive gas comprising the other of (a) carbon or (b)boron and nitrogen; f) exciting said second reactive gas in saidreaction chamber by applying thereto a DC biased, RF electric field; andg) depositing a film formed from said second reactive gas.
 13. Themethod as defined in claim 12, wherein said stacked structure is a superlattice structure.
 14. The method as defined in claim 12, wherein saidfirst reactive gas includes carbon.
 15. The method as defined in claim12, wherein said first reactive gas includes boron and nitrogen.
 16. Themethod as defined in claim 12, further comprising continuing steps b-guntil a number of films are formed.
 17. A method of forming a stackedstructure for a substrate, said method comprising the steps of:a)disposing a substrate in a reaction chamber; b) inputting into saidreaction chamber a first reactive gas comprising one of (a) carbon or(b) boron and nitrogen; c) exciting said first reactive gas in saidreaction chamber by applying microwave electromagnetic energy thereto inthe presence of a magnetic field in the pressure range of 0.01-300 Torrto cause cyclotron resonance in the reaction chamber; and d) depositinga film formed from said first reactive gas; e) inputting into saidreaction chamber a second reactive gas comprising the other of (a)carbon or (b) boron and nitrogen; f) exciting said second reactive gasin said reaction chamber by applying biased DC or RF microwaveelectromagnetic energy thereto in the presence of a magnetic field inthe pressure range of 0.01-300 Torr to cause cyclotron resonance in thereaction chamber; and g) depositing a film formed from said secondreactive gas.
 18. The method as defined in claim 17, wherein saidstacked structure is a supper lattice structure.
 19. The method asdefined in claim 17, wherein said first reactive gas includes carbon.20. The method as defined in claim 17, wherein said first reactive gasincludes boron and nitrogen.
 21. The method as defined in claim 10,further comprising continuing steps b-g until a number of films areformed.